The demand for very low bit-rate speech coders (2.4 kb/s and below) has increased significantly in recent years. Applications for these coders include mobile telephony, internet telephony, automatic answering machines and military communication systems as well as voice paging networks. Many speech coding algorithms have been developed for these applications. These algorithms include: Mixed Excitation Linear Prediction Coding (MELP), Prototype Waveform Interpolation Coding (PWI), Sinusoidal Transform Coding (STC) and Multiband Excitation Coding (MBE). In all of these algorithms, only the magnitude information of an LP filter residual signal or a speech signal is transmitted. In use of these algorithms, the phase information is recovered at the decoder by modeling, or simply omitted.
However, omitting phase information in this way results in a synthetic and "buzzing" quality in the decoded speech. Although phase information may be derived from the encoded magnitude spectrum using Sinusoidal Transform Coding, synthetic and "buzzing" qualities still exist in the decoded speech owing to minimum phase assumptions in the speech production model. Improved speech quality has been reported when the phase spectra of some pre-stored waveforms are used, but only a little information from the pre-stored waveforms is revealed using this technique.
It is an object of this invention to provide a method and apparatus for reconstructing a linear prediction systhesis filter excitation signal, for use in speech processing, wherein the above mentioned disadvantages may be alleviated.